This invention concerns timepieces including a case formed at least partially from a hard natural or artificial mineral material such as granite or semi-precious stones, or, such as carbides or nitrides of certain metals as for instance tungsten or titanium.
In U.S. Pat. No. 3,242,664 there is described a timepiece the case of which includes an inner metal caseband to which is fitted a crystal and a back cover surmounted by a protective cap. The latter is formed from a hard material by sintering a powder based on a metallic carbide. The inner caseband is glued to the cap basically in order to compensate for variations in the cap tolerances, it being well known that the dimensions thereof cannot be obtained precisely in view of the considerable shrinkage which occurs during the sintering operation.
Should it be wished to replace the cap of sintered material in this particular case by a cap of hard stone there must be machined in the latter a central opening to enable assembly of the crystal onto the inner metallic caseband and evidently likewise to enable observation of the dial which is also fastened onto this inner caseband. This arrangement renders such substitutions extremely difficult since machining of hard stone is obtained by grinding, this being a machining operation during which only relatively small amounts of material may be removed in a given time period. To this must be added that the central opening weakens substantially the cap from whence there arises major risks of breakage during the machining and assembly operations of the timepiece. Thus should one undertake such substitution one may also be confronted with a high rate of rejects having as consequence a relatively high manufacturing price.
On the other hand in view of the presence of the metallic ring forming the inner caseband one finally obtains a timepiece of high rigidity with effective protection of the case against wear by virtue of the cap.
In sum from this analysis of U.S. Pat. No. 3,242,664 it may be determined that the case described therein is mediocre as to its rigidity during the course of manufacture while such rigidity is satisfactory to the timepiece user.
Finally it may be noted that the weakness of the cap will be a negative factor during manufacture whether it be formed of sintered material or machined stone.
U.S. Pat. No. 4,075,828 provides a teaching which contributes to perfecting the assembly described in U.S. Pat. No. 3,242,664 in the sense that it recommends employment of a piece of machined semi-precious material including a caseband lacking a central opening, which in this case remains blocked by a curved surface formed entirely in one piece with the caseband and serving as a dial for the timepiece. This part which is of relatively simple form requires however machining of the movement housing, of the space between the attachment lugs and above all of several through holes intended to accomodate the fastening feet. Effectively it is by such feet that a bezel intended for fastening the crystal and a back cover is applied to the caseband, the feet cooperating with nuts screwed thereon and embedded in the thickness of the back cover. Anti-moisture packings are squeezed against the two faces of the caseband respectively by the bezel and the back cover.
Although this arrangement provides certain advantages in respect of manufacture of the part forming as one single piece the caseband and the dial, it nevertheless remains necessary to carry out a delicate machining operation on the stone, namely piercing the passage holes for the fastening feet. The presence of these holes can moreover also cause cracks leading to breakage in the stone either during machining or when worn, whenever the part undergoes shocks. Such difficulty evidently may be overcome but only at the price of increasing the thickness of the stone part which in turn increases the overall thickness of the timepiece.
Furthermore in the arrangement of this patent the movement is held in its housing by a lateral groove formed in the wall of the housing. This groove likewise necessitates a delicate machining operation in the stone. Such fastening method also implies that the movement be elastically supported against the bottom of the housing (i.e. on the central face of the curved surface opposite the dial). Consequently when the timepiece undergoes shocks while being worn the inertia of the movement acting against this curved surface brings about a risk of starting ruptures at the junction point thereof with the caseband.
Finally it is to be noted that this assembly comprises numerous components and employs packings applied to the faces of the stone whereby it is not possible to guarantee perfect anti-moisture sealing in view of the nature of the stone material.